The invention described herein relates to nuclear reactor fuel assemblies and more particularly to a fuel assembly grid designed to improve reactor performance and to be manufactured at a cost less than conventional grids.
Commercial nuclear reactors used for generating electric power include a core composed of a multitude of fuel assemblies which generate heat used for electric power generation purposes. Each fuel assembly includes an array of fuel rods and control rod guide tubes held in spaced relationship with each other by grids of egg-crate configuration spaced along the fuel assembly length. The fuel rods may be approximately 0.5 inch in diameter and about 14 feet long thus requiring a number of supporting grids along their length. Each grid includes interwoven Inconel or Zircaloy straps which form multiple cells, each cell having springs on two adjacent walls and a pair of projections or dimples on each of the other two walls forming a cell. The springs laterally impress resistive forces on each fuel rod in the assembly. Although this fuel assembly design performs exceptionally well in a nuclear reactor, one disadvantage inherent in the design is that the the inwardly projecting springs and dimples occasionally mar or score the surface of fuel rods during the time they are being pulled into the fuel assembly grids. In carrying out this fuel rod loading operation, the grids are held immovably in position while a longitudinal steel rod attached to the end of a fuel rod pulls it axially through the aligned openings or cells in the grids. As the rod engages the springs and dimples in the grid cells, their edges engage the exposed relatively soft surface of the moving fuel rod and, in some cases, score its surface sufficiently deep as to cause the rod to fall outside established fuel rod surface specifications.
Also, the grid strap material from which the springs and dimples are formed has a high neutron capture cross section, particularly when made of Inconel. Although annealed Zircaloy is not as deleterious, to some extent it adversely affects reactor performance and efficiency. The best balance between material stiffness and low neutron capture cross section should be reached for efficiency purposes.